Geomorphological and Sedimentological Comparison of Fluvial Terraces and Karst Caves in Zhangjiajie, Northwest Hunan, China: An

Environ Earth Sci (2011) 64:671–683 DOI 10.1007/s12665-010-0887-6

ORIGINAL ARTICLE

Geomorphological and sedimentological comparison of ﬂuvial terraces and karst caves in Zhangjiajie, northwest Hunan, China: an archive of sandstone landform development

Guifang Yang • Xujiao Zhang • Mingzhong Tian • Yamin Ping • Anze Chen • Zhiliang Ge • Zhiyun Ni • Zhen Yang

Received: 3 January 2010 / Accepted: 27 November 2010 / Published online: 18 December 2010 Ó Springer-Verlag 2010

Abstract The Zhangjiajie Sandstone Peak Forest Geo- landscape. The beginning of the sandstone landscape devel- park (Zhangjiajie World Geopark) of northwest Hunan, opment must be earlier than the aggradation of the ﬂuvial

China hosts a well-preserved sequence of fluvial terraces terrace T4, allowing this unique landscape to occur in the and karst caves. In this contribution, a comparative study of Middle Pleistocene. fluvial terraces with karst caves along the middle-lower Suoxi River in Zhangjiajie World Geopark is presented to Keywords Fluvial terrace Karst cave Sandstone improve the understanding of the development of striking landscape Geomorphic evolution Zhangjiajie sandstone landscape in the upper Suoxi River. By integrating geomorphological, sedimentological, and geochronological techniques, the possible correlation between Introduction fluvial terraces and karst caves, as well as their climatic and tectonic implications is investigated. The available The geomorphic features of the quartz sandstone landscape electron spin resonance and thermo-luminescence numeri- in Zhangjiajie Sandstone Peak Forest Geopark (or Zhang- cal ages coupled with morphostratigraphic analysis indi- jiajie World Geopark), northwest Hunan, China, have been cate that aggradation of fluvial terrace levels occurred at roughly described through the comparative analysis with ca. 347 ± 34 ka (T4), 104.45 ± 8.88 to 117.62 ± 9.99 ka adjacent regions aiming to determine their connection with (T3), 60.95 ± 5.18 ka (T2), and Holocene (T1), followed climatic control and surface uplift (Guo 1982; Chen 1988; by the stream incision. Fluvial terrace levels (T4 to T1) Hunan Geo-environmental Monitoring Center 1988; Deng correlate morphologically with the karst cave levels (L1 to 1989; Chen 1993; Wu and Zhang 2002; Tang et al. 2005). L4), yet the proposed chronology for the fluvial terrace Recent studies have been focused on the timing and geo- levels is a bit later than the chronological data obtained morphic evolution of the sandstone landscape in the upper from karst caves. In northwest Hunan, where a unique reach of the Suoxi River in Zhangjiajie World Geopark sandstone peak forest landscape was extensively developed, through the analysis of fluvial terraces and karst caves with the fluvial terrace sequences as well as the cave systems are the a wide distribution in the middle and lower reaches of the important archives for studying the evolution of the sandstone Suoxi River (Fig. 1a). However, prior to this study, very little was known about the chronology of these morphogenetic sequences, which could be very useful for the reconstruction and interpretation of the geomorphic evo- & G. Yang ( ) X. Zhang M. Tian Y. Ping Z. Ge Z. Ni lution of the unique sandstone landscape in the study Z. Yang School of Earth Sciences and Resources, area (Yang et al. 2009). Difficulties arise when dateable China University of Geosciences, Beijing 100083, China landforms and deposits are poorly preserved or even e-mail: [email protected] unavailable, as it is often the case in many geological settings, such as the exposed sandstone areas. The stepped A. Chen Chinese Academy of Geological Sciences, morphostratigraphic sequences in the study area suggest Beijing 100037, China that integrated geomorphological, sedimentological, and 123 672 Environ Earth Sci (2011) 64:671–683

Fig. 1 a The satellite image of Zhangjiajie showing the spatial distribution of sandstone landscapes, karst caves and ﬂuvial terraces in the upper, middle, and middle-lower reaches of the Suoxi River; b geomorphic sketch map of Zhangjiajie Sandstone Peak Forest Geopark indicating the major landform areas in the study region

geochronological analyses may provide valuable informa- geomorphological, sedimentological, and geochronological tion on the evolution of the sandstone landscape in the techniques, the possible correlation between terraces and study area. karst caves, as well as their climatic and tectonic impli- For a long time it was believed that fluvial terrace cations can be investigated. These studies allow recon- staircases or multileveled karst caves could indicate former structing the evolution of depositional systems and offer river bed levels that would provide field evidence for long- valuable insights into the basin geomorphic changes in term evolution of river catchments (e.g., Bull 1990; Bourne response to allocyclic factors (Maddy et al. 2001; Robus- and Twidale 2000; Bridgland 2000; Maddy et al. 2000; telli et al. 2005, 2009). Piccini et al. 2003). In recent years, there is growing Given the specific geological and climatic settings of the awareness of the importance of stepped morphogenetic study area, the present study reports the results of geo- sequences because of their potential climatic and tectonic morphological and sedimentological investigation analyz- significance (Bridgland et al. 2004; Peulvast and Sales ing the relationships between fluvial terraces and karst 2004; Gao et al. 2005, 2008; Westaway et al. 2006; Brid- caves in the Suoxi River basin from Zhangjiajie Sandstone gland and Westaway 2008; Carcaillet et al. 2009; Claessens Peak Forest Geopark to adjacent areas. The aim of this et al. 2009; Robustelli et al. 2009; Strasser et al. 2009; work is to characterize the fluvial terraces and karst caves, Westaway 2009, 2010). Chronologically, dating is a key relating them to the formation and geomorphic evolution of to link terraces with karst caves. By integrating the unique sandstone landscape of Zhangjiajie. Numerical

123 Environ Earth Sci (2011) 64:671–683 673 age data provide information on the timing of sandstone In the study area, the drainage system is controlled by the landform development in Zhangjiajie World Geopark. Suoxi River, a second-order tributary of the Lishui River, which ﬂows eastward through the region. Of the approximately 70 km long segment of the Suoxi Valley studied Regional setting here, the upper 20 km stretch lies in the core part of Zhangjiajie Geopark whereas the remaining 50 km lies Zhangjiajie Sandstone Peak Forest Geopark (29°1301800– downstream. It has dissected the quartz sandstone bedrock 29°2702700N, 110°1800000–110°4101500E) is located in the forming various sandstone topographies at different heights northwestern sector of Hunan Province, China, covering a varying from 1,000 m a.s.l. to the present-day river bed. total area of 398 km2 (Fig. 1). Elevation ranges from Zhangjiajie Sandstone Peak Forest Geopark is just in the *200 to 300 m a.s.l. in the valley bottom to ca. 1,300 m transition zone between the northeastern Yun-Gui Plateau a.s.l. in the mountain peaks (Fig. 2). The area is charac- and the middle-low mountain area of northwestern Hunan terized by a plateau-type monsoon climate, receiving an Province. Stratigraphic deposits range from youngest annual average precipitation of 1,400 mm and with an Holocene to Silurian and Devonian age that are represen- annual average temperature of 16°C (Hunan Geo-envi- tative of the region (Hunan Bureau of Geology and Mineral ronmental Monitoring Center 1988). Resources 1988; Fig. 2). Strata of the Middle Silurian The geopark stands across the Jiangnan Oldland and occur in the south, north, and west of the geopark and Yangtze Platform. The area can be considered as relatively constitute the bedrock foundation underlying the sand- stable from the tectonic point of view and magmatic stone. The Silurian strata is disconformably overlain by the activity is absent (Fig. 2). Physiographically, the study area approximately 520 m thick Middle Devonian Yuntaiguan is characterized by a slightly undulating terrain with a Formation (D2yn), which is dominated by gray-white, fresh general slope of 5° to 8° towards the NE–NNE direction pink, thick or medium-thick ﬁne-grained quartz sandstone (Hunan Geo-environmental Monitoring Center 1988). with a series of interbedded thin-layer siltstones and muddy

Fig. 2 Geological map and cross-section of Zhangjiajie World Geopark (modiﬁed from Hunan Geo-environmental Monitoring Center 1988)

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siltstones. The overlying Huangjiadeng Formation (D3h)of During sampling, iron tubes 10-cm in diameter were driven the Upper Devonian has a regional thickness of 5.3–40.6 m into the fluvial profiles and sealed immediately after sam- and is a fine-grained ferruginous quartz sandstone, with ple collection. In the dating lab, the laboratory technician 1–3 layers of oolitic hematite on the top, making the removed both ends of the samples to yield reliable resistant red hats on some sandstone peaks. The Huangj- numerical ages. The TL dating was performed in the iadeng Formation sandstone is unconformably overlain by Institute of Geology, China Earthquake Administration thick Permian limestone, with Carboniferous strata being (CEA), whilst the ESR dating was examined in the Institute normally absent. In the geopark, the sandstone peak forest of Crustal Dynamics, CEA. The general principles and landforms are mostly developed in the Upper and Middle experimental protocols of the ESR and TL dating methods

Devonian units (D2?3), whereas limestone karst landforms for deposits from fluvial terraces and highest level of Hu- appear in the Permian strata in northeast Tianzi Mountain anglong Cave are based on previous studies (Lu et al. 1987, above the peak forest landforms or in the Triassic strata 1988; Bahain et al. 2007; Yin et al. 2007). According to along the Suoxi River valley (Fig. 2). The Quaternary prior works, the systematic error of ESR and TL dating was deposits are very limited, usually represented by fluvial estimated to be within 10% (Bahain et al. 2007; Yin et al. deposits. 2007). In order to compare the chronology of different terraces with cave levels, the weathering rinds and sedimentary features of the gravels from the highest level of Materials and methods Huanglong Cave were investigated using the method of field gravel counts, including determining the weathering Two field surveys were carried out in October 2008 and rinds and colors, lithology, orientation, size and roundness May 2009 to examine the various stepped morphogenetic of the clasts in conglomerates (cf., Laming 1966). At least sequences developed in the Zhangjiajie World Geopark 116 pebbles and cobbles were measured, allowing for the and its adjacent terrains (Fig. 1). Multidisciplinary geo- statistically valid results. morphological, sedimentological, and geochronological approaches have been applied to better understand the evolutionary history of the striking sandstone landscape. Description of fluvial terraces and karst caves The thickness and elevation of the terraces above the present river bed have been measured by means of GPS, Fluvial terraces large-scale topographic mapping and field surveys. The height, length, and structural configuration of the caves In the study area, a stepped sequence of fluvial terraces were systemically measured and analyzed in detail during occurs along the Suoxi River in Zhangjiajie, northwest the investigation. The lithofacies were identified and Hunan Province, China. By integrating the interpretation of broadly grouped into gravel, sand, and clay, which was large-scale topographic maps, chronologic analyses, and further divided into sub-facies based on textural maturity, field surveys, we distinguished four terrace levels in the stratigraphication, and color. A total of seven bulk samples Suoxi River. These terrace levels are labeled according to was collected from silt-sized deposits of fluvial terraces their relative topographic position from the highest (T4)to and karst caves along the Suoxi River valley, as well as the the present-day floodplain (Figs. 3, 4). adjacent Maoxi River. The geochronology of terrace and In the upstream stretch, one terrace level has been karst cave deposits was established by means of thermo- observed on both banks of the Suoxi River. This terrace luminescence (TL) and electron spin resonance (ESR) level is present from 275 to 280 m a.s.l. and *10–12 m analyses based on previous publications (Forman 1989; above the Suoxi River bed, becoming continuous in

Laurent et al. 1998; Chiavari et al. 2001; Wray et al. 2001; downstream areas (Figs. 3a, 4). The ﬂuvial terrace T1 is a Tissoux et al. 2008). Two quartz-rich samples were strath cut across the Triassic limestone. Its surface is selected from the highest terrace and karst cave level for covered by some gravel deposits, which occur in patches ESR dating, and ﬁve quartz-rich samples were collected on the left/north bank of the Suoxi River and extend in the from lower terrace levels for TL dating, respectively downstream direction around Dengjiaping (P01, Fig. 3a).

(Table 1). The samples were mostly composed of sand and In the lower reaches of the Suoxi River, the T1 terrace silt and they were collected from 10–15 cm from the lies at about 7 m above the present-day river bed (ca. exposures in the fluvial terraces along the Suoxi and 173 m a.s.l.). At places, this terrace level is predominant by adjacent Maoxi Rivers and highest level in Huanglong a simple dual-texture, with 2–3 m thick finer alluvial Cave. Proximity to boulders was avoided of the removing deposits resting on the 1–2 m thick gravel layer (Fig. 3d). and a distance of 20 cm was considered sufficient to The T2 terrace consists of a lower faces of gravel layer minimize possible contamination of the sample material. 3–5 m in thickness and an overlying faces of alluvial silts 123 Environ Earth Sci (2011) 64:671–683 675

Table 1 Age of Huanglong Geomorphic feature Level Elevation/ Above river Age Cave and ﬂuvial terraces from m a.s.l. level/m a.s.l. the Suoxi, Maoxi, Yangtze, Yellow, and Weihe rivers Huanglong Cave (by authors) L1 *345 80–90 463 ± 46 ka (ESR) 2 3 L2 *300 40–45 Qp–Qp 3 L3 *280 20–25 Qp

L4 *262 2–5 Qh

Terraces of middle Suoxi River T4 238 *35–40 (by authors) T3 230 *25–30 104.45 ± 8.88 ka (TL) 117.62 ± 9.99 ka (TL)

T2 220 *15–20

T1 207 *4–6

Terraces of lower Maoxi River T4 256 78–83 347 ± 34 ka (ESR) (by authors) T3 235 57–62 151.05 ± 12.84 ka (TL) 201.24 ± 17.11 ka (TL)

T2 206 28–33 60.95 ± 5.18 ka (TL)

T1 188 10–15

Terraces of the middle Yangtze River T4 0.3–0.5 Ma (Li et al. 2001) T3 0.11–0.15 Ma

T2 0.05–0.06 Ma

T1 0.01–0.03 Ma

Terraces of the Weihe River T4 0.412 Ma (Gao et al. 2008) T3 0.128 Ma

T2 0.064 Ma

T1 0.011 Ma

Terraces of the Yellow River T4 0.6 Ma (Li et al. 1997) T3 0.120 Ma

T2 0.055 Ma

T1 0.010 Ma

Fig. 3 Integrated cross-section depicting various terrace levels on both banks of the upper (a) to middle (b) and lower reaches (c) and (d) of the Suoxi River

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Fig. 4 Longitudinal proﬁle with west–east direction of ﬂuvial terraces in the Suoxi River Basin

0.5–1 m in thickness. This terrace is represented by small of the Zhangjiajie World Geopark (Chen 1987; Ge et al. treads ranging from 178 to 183 m a.s.l. Remnants of terrace 2009). The cave network ranges from 260 to 400 m a.s.l.,

T2 are continuous in downstream areas of the Suoxi River with exit standing approximately 5 ± 2 m above the with a tread at 180 m a.s.l. The T3 terrace level is repre- present Suoxi River. Huanglong Cave is characterized by a sented at *22–25 m above the Suoxi River bed in some unique configuration and wide variety of stalactites, sta- reaches of the Suoxi River valley by a 1–1.5 m thick gravel lagmites, columns, and flowstones (Cheng 1988; Yang layer, overlain by 0.2–0.4 m of alluvial silts. The fluvial 2007; Ge et al. 2009; Fig. 6). The cave can be divided into succession comprise dominantly well-sorted, weakly four different east–west trending levels, with a total length weathered, and moderate imbricated sandstone or siltstone, of 13 km and a vertical development of 140 m. which was stuffed by relatively loose and light brown The upper level ranges from 345 to 400 m a.s.l., sands and silts (Fig. 5). Clast size ranges from 10 to 20 cm including Dragon King’s Palace and the Stone-flower (along a axis) but the maximum size measured up to Chamber (Fig. 6). A 60-cm thick water-laid deposits *50 cm. The matrix is light olive brown, coarse sand and composed of clayey sand and gravels have been recognized grit. In the middle reach of the Suoxi River valley, terraces (Figs. 6, 7). The gravel diameter is dominantly 3–5 cm, show similar characteristics in terms of relative height and reaching up to 18 cm and showing relatively good sorting sedimentary features, whereas having a higher absolute and roundness though with poor fabrics and a weak degree height (Fig. 3b, c). These three terrace levels can be traced of weathering (Figs. 6, 7). The sedimentary succession downstream and at P06, they occur at *9, 24 and 39 m exhibits a characteristic and fairly simple admixture of above the Suoxi River bed (Figs. 3c, 4). Additionally, some resistant sandstone/siltstone clasts floating in reddish- small relics of fluvial deposits occur at higher elevation in the brown mud matrix. For instance, among the 116 pebbles peak summits along the middle Suoxi River valley (Figs. 3b, and cobbles of water-laid deposits, 31% were milky quartz 4). When taking the relative height from the Suoxi River siltstone, 29.3% grayish yellow quartz siltstone, 14% light bed and spatial distribution pattern into consideration, fresh red Quartz siltstone, 10% grayish white quartz silt- these deposits might be equivalent to the terrace level T4, stone, 9% purple quartz siltstone, and a small amount of extending laterally for several kilometers (Figs. 3b, 4). brick-red siltstone, light brown quartz siltstone, and sage

Two samples from the T3 terrace level were collected green quartz siltstone (Fig. 6). Infilling fine-grained sand, for TL dating (Fig. 3b), yielding 104.45 ± 8.88 and silt, and even clay of matrix materials are strongly 117.62 ± 9.99 ka (Table 1), in agreement with regional cemented and very tough indeed. The ESR dating of the results of 0.1–0.2 Ma from the adjacent Maoxi River highest gravel layer found in Huanglong Cave indicate that (151.05 ± 12.84 ka and 201.24 ± 17.11 ka, Table 1) and the formation of the cave started more than 463 ± 46 ka middle Yangtze River (Xie 1991; Li et al. 2001; Yang ago (Table 1). 2006). Assemble Platform and Immortal Hall, with an approximate elevation of 300 m a.s.l., constitute the second Typical karst cave: Huanglong Cave level. Water-laid deposits including silt, sand, and pebbles have been found at the junction of Dragon Ballroom and Ji Huanglong Cave is located 10 km east of Jundiping village Platform. This level is the one that reached the highest and in the northern side of the Suoxi Valley, forming part length. The third floor is represented by the Charming

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Fig. 5 Simpliﬁed vertical section of terrace level T3 in P05 along the Suoxi River

Fig. 6 Horizontal (a) and vertical (b) distribution of Huanglong Cave (shaded colors used to differentiate different levels; modiﬁed from Hunan Geo-environmental Monitoring Center 1988)

Palace situated at 280 m a.s.l. and 20–25 m above the becoming the lowest level of the karst cave. The present present river channel (255–260 m). On the west of the cave underground river, with the same elevation as the T1 ter- corridor, the water-laid deposits, mainly consisting of race level in the upper Suoxi River (Chen 1987), maintains sandy clay, sand, and gravels are 1.5–2.0 m thick (Fig. 6). a hydraulic connection with the Suoxi River by supplying The detrital sediments from Yanba Grotto to Charming and receiving water in the low-ﬂow and ﬂood seasons, Palace are dissected by a modern underground river, respectively.

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Fig. 7 Gravel layer and its characteristics in the highest level of fresh, weak, moderate and complete-weathering, respectively; Huanglong Cave (a) and (b) general and close-up views of gravel (e) Lithologic component: 1, light red quartz siltstone; 2, milky layer in the highest level in Huanglong Cave; c Roundness: 1–5 quartz siltstone; 3, purple quartz siltstone; 4, light brown quartz represent the extremely well-, well-rounded, rounded, subangular and siltstone; 5, grayish white quartz siltstone; 6, grayish yellow quartz angular geometry of clasts; d weathering degree; 1–4 indicate the siltstone; 7, sage green quartz siltstone; 8, brick-red quartz siltstone

Fig. 8 Major karst caves and stepped morphogenetic sequences developed during the middle Mid-Pleistocene in China

Discussion development. During the Quaternary, however, with glacial-interglacial cycles accompanied by tectonic uplift, the Terraces ages and correlation with karst caves conditions were adequate for developing stepped morphogenetic sequences (Cheng 1988; Hunan Geo-environmental In the study area, the relatively hot and dry Neocene cli- Monitoring Center 1988; Bourne and Twidale 2000;Ge mate was not favorable for alluviation process and cave et al. 2009; Yang et al. 2009). As noted in many other

123 Environ Earth Sci (2011) 64:671–683 679 studies, episodes of karst cave and terrace aggradation can the ﬂuvial terraces. Some remnants of this level have been be associated with periods of interglacial and glacial, while identiﬁed in the middle reach of this valley during the river incision is more intensive during glacial/interglacial investigation, the typical dual stratigraphy and pebble im- transitions (Bridgland 2000; Piccini et al. 2003; Bridgland brications, however, cannot be recognized (Fig. 3). Only in and Westaway 2008). some restricted low-relief areas there are small to medium- The Huanglong Cave initially occurred more than scale remnants of this level (Figs. 3b, 4).

463 ± 46 ka ago, broadly contemporaneous with MIS 11, The altitude of the widespread and continuous T3 terrace during interglacial conditions dominated by the wettest level in the adjacent Maoxi River and the most extensive climate. The warm and humid climatic conditions during level 2 of Huanglong Cave (Ge et al. 2009) indicate the the Middle Pleistocene would be favorable for cave paleoclimatic condition during they were formed. The development (Lu 1986; Chen 1992). In regional terms, a climate of MIS 7 during the level 2 of karst cave developed large number of caves developed during this time from must be extremely warm and moist in this area. This par- southern to northern China (Lu 1986; Chen 1992; Wang ticular period of MIS 7e dominated by extremely warm and et al. 2002; Ge et al. 2009; Zhang et al. 2009; Fig. 8). This humid climate would favour the extensive karstification was followed by the aggradation of terrace level T4 in the development. On the basis of regional correlations, the adjacent Maoxi River and middle Yangtze River (0.46 ± well-developed terrace level T3 has been assigned to MIS 0.046 Ma, 0.347 ± 0.034 Ma, and 0.3-0.5 Ma, respec- 6. Given the abundance of surface cobbles and pebbles tively; see Li et al. 2001; Xiang et al. 2005; Yang 2006; observed, the relatively weak weathering rinds of the Table 1). Four horizontal or slightly inclined cave passages gravels (with an average thickness of approximately provide valuable evidence to reconstruct the former \0.6 cm) in terrace level T3 from the Suoxi River and the groundwater table and its relation to the regional fluvial adjoining Maoxi River are considered to represent deposits base level, indicating ages of MIS 11, 7e, 5e and 3, of MIS 6, during which the dominant cool the dry climate respectively. was likely resulted in the deposition of coarse sedimentary In the present study, TL dating of five samples from the units. When the finer matrix, mainly slit and clay, enter Suoxi and the adjacent Maoxi River valleys offer an simultaneously into the alleviation during interglacial opportunity to develop a chronologically constrained evo- times, they will aggradate atop the coarser groups, pro- lutionary history for the Zhangjiajie area during the Late ducing to the dual stratigraphy. In regional correlations, the

Quaternary, with terrace levels T3 and T2 formed at ca. numerical ages, the relative height and sedimentological 0.1–0.2 Ma and 0.06 Ma, respectively (Table 1). The features of terrace level T3 indicate that it was formed positions of T3 and T2 levels above the Suoxi River suggest during the Late Pleistocene in both Maoxi and Suoxi that they were most likely developed during the Late Rivers. Similarly, the relative height and geomorphic

Pleistocene, during MIS 6 and 4, and controlled by the conﬁguration of terrace levels T2 and T1 suggest that they middle Yangtze River (Yang and Chen 1988; Xie 1990, are correlative to the two lower levels of Huanglong Cave, 1991; Tian et al. 1996; Li et al. 2001; Xiang et al. 2005; Late Pleistocene to Holocene in age. Yang 2006). The gravel layer identiﬁed in the Huanglong Cave

The good correlation between terrace level T4 and the dominated by siltstone can provide some clues on the upper cave level indicate that karst cave was formed in the origin of the cave. The completely different lithology middle Middle Pleistocene or even earlier. A significant between the gravel layer and the host rock indicates an base-level standstill is likely to have occurred during this allochthonous source for the gravel (Cheng 1988; Huang period, at which time a new karst feature emerged at an et al. 2006). We deemed that the fairly uniform lithology of elevation approximately 80–90 m above the contemporary water-laid gravels with a good sorting and roundness river level in Zhangjiajie, followed by the aggradation of should be conveyed by the river flow from the local terrace level T4 during a cold period (Table 1). Devonian sandstone outcrops, where large amounts of detritus would have been supplied by mass wasting and Geomorphological and sedimentological comparison stream erosion (Yang et al. 2009; Fig. 7). This conclusion of fluvial terraces and karst caves is also confirmed by other lithological data (Hunan Bureau of Geology and Mineral Resources 1988; Huang et al. The highest terrace level is not well preserved along the 2006; Tables 2, 3). The poor fabrics and the lack of a dual Suoxi River, because the high relief and climatic condi- stratigraphy associated with gravel layer give evidence of tions (i.e., high temperature excursions and rainfall up to the larger water flow and prompt deposition process. It is 1,400 mm/year) favor rapid erosion over a longer time probable that rainfall was abundant, perhaps confined to a span. Mechanical erosion must have been very active short reason, but coming as occasional violent cloudbursts; during the middle Middle Pleistocene, destroying most of these would have been sufficient to cause torrential flow in 123 680 Environ Earth Sci (2011) 64:671–683

Table 2 Host rock component of Huanglong Cave (unit: %; revised from Cheng 1988) Host rock Calcite Clay Ferric ingredient Dolomite Micro-ﬁssure

Fine-grained silty limestone 98 2–3 1 Minority Dolomitic limestone 86 0.5–1.0 11 1.0–2.0

Table 3 Lithologic components of different Zhangjiajie sandstone landscapes (unit: %; modiﬁed from Huang et al. 2006)

Locality Lithology Stratum SiO2 CaO Al2O3 TiO2 Fe2O3 Others

Baofeng Lake White quartz sandstone D2yn 97.20 0.00018 1.84 0.15 0.00076 0.0015

White quartz sandstone D2yn 94.20 0.00039 1.66 0.00063 0.0004 0.0007

Jinbian stream White Quartz sandstone D2yn 95.90 0.71 2.18 0.13 0.14 0.13053

White quartz sandstone D2yn 94.00 0.51 2.94 0.23 1.25 0.00058

Baizhang Valley Red quartz sandstone D2yn 83.60 0.00066 10.60 0.36 2.67 0.00127

Red silty mudstone D2yn 67.20 0.00011 22.00 0.92 2.17 0.00055

Xianyao bay Red quartz sandstone D2yn 79.40 0.12 4.77 0.39 13.20 0.000132

White siltstone D2yn 66.80 0.00021 24.70 1.30 0.27 0.00092

Koho Red quartz sandstone D3h 92.20 0.00097 5.26 0.21 0.46 0.33

Peak forest Red quartz sandstone D3h 93.50 0.00062 0.88 0.21 3.96 0.24019

D2yn Yuntaiguan Formation of Middle Devonian, D3h Huangjiadeng Formation of Upper Devonian valleys. Material of all sizes would be moved and carried each other very well since they are commonly controlled relatively long distances. When transported into the cave, by the Suoxi River (Fig. 9). The top level of Huanglong the pebbles and cobbles were deposited quickly in a rela- Cave corresponds to the highest level (L1) of sandstone tively closed environment. Due to the lack of exposure to landform and the highest terrace level of the Suoxi River, the surface, weathering process on gravels was limited while the lowest river water levels of cave and fluvial (Fig. 7). In this case, the medium to weak weathering rinds systems should represent the current standstill level. The of most gravels (mostly \0.8 cm) indicative of Late good correlations in geomorphic pattern among these Pleistocene or even in late Middle Pleistocene times may stepped morphogenetic sequences suggest that the warm post-date the deposition (Fig. 7d). The saturated infilled and moist climate during middle Middle Pleistocene finer sediments with widespread rubefaction are evidence (*46.3 ka) favored karstification processes, followed by of onetime presence of a warm and humid climate during the aggradation of terrace level T4 during a cold period which they were formed, in consistence with the paleo- (Table 1). climatic picture of the study area over the middle Middle Therefore we assume the beginning of incision is a bit

Pleistocene (Hunan Bureau of Geology and Mineral earlier than the aggradation of terrace level T4 Resources 1988; Liu 2002). From this it may be deduced ([0.35 Ma) on the scale of the Suoxi River. During an that the Huanglong Cave preferably occurred in thus a early stage, the Suoxi River might have begun to develop subtropical climatic regime in Zhangjiajie during the its drainage network, with the fluvial dissection of river middle Middle Pleistocene. into the sandstone bedrock within the Zhangjiajie Sand- stone Peak Forest Geopark. The regional tectonic move- Implication for sandstone landform development ment and associated stress release lead to highly developed fracture and joint systems in the study area, The sandstone landforms concentrate mainly on the which generates the mass wasting and greatly accelerate boundary of Tianzi Mountain and areas between Wangjia the stream incision process (Chen 1987, 1988;Yangetal. village and Heicao Valley in the upper reach of the Suoxi 2009). For these reasons, the timing of sandstone land- River (Fig. 1b). Their summits show four distinctive ero- form formation, however, should be constrained to Mid- sional levels varying primarily from 800 to 1,000, 700 to dle Pleistocene when the long-term tectonic stable period 750, 500 to 550 and 300 to 400 m. a.s.l., respectively was finished. (Fig. 9a). When comparing the typical sandstone land- The chronologic data from the karst cave and terrace scapes with those of fluvial terraces and karst caves, four sequences in the Suoxi and its adjacent Maoxi rivers also distinctive geomorphic levels are evident and correspond to helps to understand the evolution of sandstone landform.

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Fig. 9 Geomorphic correlation of the sandstone peak forest landscapes (a), karst caves (b) and ﬂuvial terraces (c) in the upper, middle, and middle- lower Suoxi catchment from Zhangjiajie

The geochronology of the stepped morphogenetic sequen- sedimentological and geochronological features, we thus ces of the Suoxi River that formed during the middle period propose that the Suoxi River began to incise the sandstone of the Middle Pleistocene has been constrained dating landform in the Middle Pleistocene, accompanied by the terraces and karst cave systems. The integration of a geo- later mass wasting owing to the well-developed fracture morphological analysis and the available data may allow us and joint systems in the region. to propose some inferences on the age of the sandstone landscape. Consequently, the earliest possible time of stream incision of Suoxi should be earlier than the age of Conclusions the oldest terrace level T4 (approximately [347 ± 34 ka; Fig. 9). They are also supported by the additional evidence In this paper, we present the results of an integrated geo- from sedimentary features and geomorphic pattern in the morphological, sedimentological and geochronological study region. The strongly cemented gravel layer and study carried out along the Suoxi River of northwest Hunan red inﬁlled deposit features in highest level of Huanglong Province, China. This area is characterized by the occur- Cave should be dated as middle Middle Pleistocene rence of four distinct erosional levels in the sandstone when warm and humid climate was predominant. Based on landscape, karst caves and ﬂuvial terraces in the upper, these comparable evidences from geomorphological, middle, and middle-lower reaches of the Suoxi River. This

123 682 Environ Earth Sci (2011) 64:671–683 present study indicates that fluvial terraces and karst caves Carcaillet J, Mugnier JL, Koçi R, Jouanne F (2009) Uplift and active from the Suoxi River basin with comparable ages help to tectonics of southern Albania inferred from incision of alluvial terraces. Quat Res 71:465–476 understand the evolution of the sandstone landscape. Chen CM (1987) The geology-factor of the development of the karst Sandstone landscape development occurred probably in the geotto on the borderland of northwest Hunan. Nat Sci J Hunan middle Middle Pleistocene due primarily to the surface Norm Univ 10(1):97–104 (in Chinese) uplift and stream incision. These also can be generally Chen CM (1988) On the new tectogenesis of northwestern Hunan. Hunan Geol 7(2):64–72 (in Chinese) correlated with each other both temporally and spatially Chen CM (1992) Karst cave in diwas (geodepressions), China. 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